The present invention relates to a focus adjusting device facilitating adjustment of projection display images with high definition.
FIG. 1 is a configuration diagram showing arrangement of a CRT (cathode ray tube) of an electromagnetically focusing CRT display of the prior art and its peripheral components. Numeral 1 denotes a CRT face, 2 an electron gun, 3 a deflection coil, 4 an electromagnetic focus coil, 5 and 6 magnets each having two poles (N and S). By slightly rotating the magnets 5 and 6 in a plane perpendicular to the tube axis, the strength and direction of the magnetic field can be adjusted.
In FIG. 1, the bipolar magnet 5 is provided for absorbing dispersion in the direction of electrons emitted from the electron gun 2 and for making the electrons pass through the center of the focus coil 4 on the basis of Lorentz's law. The focus coil 4 forms a magnetic field of approximately 300 Gauss nearly in parallel to the tube axis to focus an electron beam. The bipolar magnet 6 is provided for absorbing dispersion in the direction of electrons outputted from the focus coil 4 and for making the electrons strike against the center of the CRT face 1 (in the undriven state of the deflection coil).
In a high definition projection display apparatus using a projection CRT as the CRT, a deflection yoke of the uniform magnetic field type is used. In that case the, sine of the deflection angle is proportionate to the deflection current in accordance with track physics of the electron beam. On the other hand, the deflection distance on the screen is proportionate to the tangent of the deflection angle because the fluorescent screen of the CRT is nearly planar. In case the deflection current has a distortion less sawtooth waveform, therefore, geometric distortion is caused on the screen. That is to say, the electron beam at the peripheral part of the screen is overdeflected in a peripheral radial direction in proportion to the cube of the deflection angle. These distortions are classified into so-called pincushion distortion (hereafter referred to as pin distortion as well) and S distortion, which are corrected by various well-known means, respectively.
Such an apparatus is described in D. G. Fink, Television Engineering Handbook, McGraw-Hill Book Company, Inc. (1957) pp. 6-58 to 6-62 and pp. 16-195 to 16-196. However, this is a technique of absorbing the aberration by moving a pole piece of the focus coil 4 slightly and mechanically. Extremely delicate adjustment work is thus required, resulting in industrial difficulty.
In the above described prior art, either means for making a correcting current flow through the deflection coil, or a method of disposing four magnets or electromagnets in the deflection yoke output section of the CRT funnel is adopted to correct the pincushion distortion.
Although theoretical analysis of the principle of the former means has been sufficiently performed, the former means has a problem that a large amount of electric power is required to make the correcting current flow.
The latter means has a simple configuration, and its cost is low. Since the theoretical analysis of its principle has not been sufficiently performed yet, however, the latter means depends a trial-and-error technique. In addition, it is difficult to correct pincushion distortion properly. Thus, the latter means has a problem that complicated image distortion remains. Further, the latter means deteriorates electron beam focusing, and means for correcting focus deterioration caused by magnets is not disclosed in the prior art. In applications wherein focus performance is regarded as important, therefore, the magnet correction scheme has not been used.
Further, the above described technique has a problem that it is difficult to reduce aberration caused by dispersion in component dimensions.
In the conventional method heretofore described, a bipolar magnet is rotated and adjusted slightly in a plane nearly perpendicular to the transit direction of the electrons to remove aberration of the electron beam. It is thus necessary to rotate and adjust the bipolar magnet while carefully observing the in-focus situation of the display image. Further, it is necessary to disposed a large mirror so as to be opposed to the display screen in order to observe the display screen. When the conventional method is used, the adjustment work itself is thus difficult to do.